red dot sight (thing)

A red dot sight is an optical sight that superimposes a tiny illuminated red dot over the point of impact on the target you are aiming at, that is of course if you have properly zeroed the sights. It comes in magnified and reflex sight configurations, reflex meaning there is no magnification.
Red dot sights usually come as a small scope that attaches to the pistol via mounting rails.

The most common brands are Tasco and Aim Point both of which are American products.

This nifty piece of hardware is quite expensive and is mostly common in IPSC open division race guns. It is not allowed in the more realistic IDPA competitions as it greatly adds to the bulk of a gun and makes it inappropriate for concealed carry.